Nowadays, with the development of the electronic industry, the power supply system is widely used for power supply. The power supply system usually includes plural power supply units joined in parallel by a system bus. Accordingly, an ORing device with an isolation diode is provided to connect between each power supply unit and the system bus for blocking the reverse current from flowing back from the system bus to the power supply unit. Further, the isolation diode is replaced by an ORing FET (field effect transistor) for lower voltage drop across the ORing device.
Generally, the ORing FET operates in either a linear mode or a saturation mode. For the ORing FET operating in the linear mode, when the output current loading decreases to no load, the gate-source voltage almost remain at the gate-source threshold voltage level, which causes the drain-source resistance of the ORing FET to remain at highest value. Thus, if the output voltage provided to the system bus is higher than the input voltage received from the power supply unit, the negative current flowing back to the ORing FET is low. However, the drain-source resistance of the ORing FET is high at heavy load, which causes low efficiency on power transmission due to more power dissipated or heated over the ORing FET.
For the ORing FET operating in the saturation mode, the gate-source voltage of the ORing FET keeps at maximum value from no load to maximum load. At heavy load, the high gate-source voltage can get low drain-source resistance on the ORing FET, which causes high efficiency on power transmission. However, when the output current loading decreases to no load, the gate-source voltage remains at maximum value and the drain-source resistance of the ORing FET remains at minimum value. Thus, when the output voltage is higher than the input voltage, the negative current flowing back to the ORing FET is high.
Therefore, there is a need of providing an ORing circuit to obviate the drawbacks encountered from the prior arts.